Procedure to reduce nitrous oxide (N2O) emissions in a combustion installation and an installation for embodiment

ABSTRACT

A procedure wherein a catalyst pertaining to the family of zeolites is injected into the vessel ( 1 ) of the circulating fluidized bed, in powder form. The installation includes a vessel ( 1 ) in which the top is connected to a separator ( 3 ) in which the bottom is connected to the vessel ( 1 ) by a recirculation loop ( 4 ), a silo ( 5 ), in which the catalyst based on crushed zeolites is stored, and which is connected by a conveyer ( 6 ) to the recirculation loop ( 4 ).

[0001] This application is based on French Application No.0003232, filed Mar. 14, 2000, which is incorporated herein by reference.

[0002] The invention concerns a procedure to reduce nitrous oxide (N₂ 0) emissions in a combustion installation which includes a vessel operating as a fluidized bed burning fuels, regardless of their state, for example coal.

[0003] Fluidized bed combustion is a developing technique for energy production. In combustion installations operating in circulating fluidized bed, it is known how to desulphurize combustion smoke by injecting limestone into the vessel. It is also known how to limit nitrogen oxide emissions (NO, NO₂) by controlling the temperature and by staggering secondary air injections in the vessel, but, in exchange, nitrous oxide emissions (N₂ 0) are increased. Yet, it is known that this gas contributes to the development of the greenhouse effect and the destruction of the ozone layer.

[0004] It is known how to reduce nitrous oxide emissions by additional combustion in combustion smoke at the outlet of a separator, of cyclone type for example, with an additional fuel, which can be natural gas or micronized coal. Furthermore, an additional air injection may be done upstream from a boiler recovering the energy of combustion smoke.

[0005] Through document DE 19728699A1, a procedure is known for catalytic reduction of nitrogen oxide NO_(x)contained in smoke, a procedure in which, after the combustion vessel, in a smoke line, ammonia and a natural zeolith catalyst are added. With this procedure, the contact between the smoke and the catalyst is short and thus requires a high volume of zeolith.

[0006] This invention offers a procedure to improve the contact time between the catalyst and the combustion gases, thus reducing the necessary volume of catalyst.

[0007] Thus, the object of this invention is a procedure to reduce nitrous oxide N₂ 0 emissions in a combustion installation which includes a vessel operating as a circulating fluidized bed characterized by the fact that a catalyst belonging to the family of zeolites is injected into the vessel of the circulating fluidized bed in powder form.

[0008] Zeolites are crystallized tetrahedric silicoaluminates. The arrangement of these tetrahedrons (SlO₂ and AlO₄) in the three directions of space, sharing oxygen, generates a microporosity consisting of channels and cavities with a size on the order of 3 to 13 Å, depending on the type of zeolith. One can incorporate into the zeolites different metallic cations, by cationic exchange, and thus give them special properties. One should preferably choose iron (Fe²⁺), which does not induce special constraints for the discharge or utilization of the ashes if the catalyst is charged into the vessel.

[0009] The catalyst is crushed to the appropriate granulometry so that it may remain a sufficiently long time in the recirculation loop of the solid particles that cross the cyclone. Tests have allowed obtaining a reduction of approximately 85% of nitrous oxide emissions by injecting a quantity of powder catalyst based on zeolites representing between 0.2 and 5% of the solid charge circulating in the vessel. Moreover, with a catalyst of average diameter ranging between 0.05 and 2 mm, the stay time in the recirculation loop is approximately 50 hours. Such granulometry also allows extracting the catalyst at the bottom of the vessel with the bed ashes, and the catalyst, so recovered, may be recycled in the vessel after being separated from bed ashes.

[0010] Thus, with the procedure according to the invention, in which the catalyst is introduced not upstream from the vessel in a smoke line, but in the vessel, one obtains a much longer duration of contact of the catalyst with the combustion gas than in the case of the prior art, cited above.

[0011] One can use synthetic or natural zeolites such as mordenite from Hungary, with Si/Al (atomic) ratio between 5 and 6.

[0012] It has also been observed that the embodiment of the procedure in accordance with the invention contributes to the conversion, in combustion smoke, of carbon monoxide (CO) into carbon dioxide (CO₂).

[0013] The embodiment of the procedure in accordance with the invention is very schematically illustrated in the single FIGURE.

[0014] In this FIGURE, a combustion installation with liquid or solid fuel, such as coal, includes a vessel operating as a circulating fluidized bed, for example. The top of the vessel is connected to a particle separator of cyclone or similar type 3. This separator receives combustion smoke at the outlet of the vessel, and includes a top part which discharges combustion smoke with flying ashes, and a bottom part, which recovers the largest solid particles 2 present in combustion smoke. The bottom of the separator 3 is connected to the bottom of the vessel 1 through a recirculation loop 4 equipped with a siphon. The powder catalyst based on crushed zeolites is stored in a silo 5, which is connected through a transport system 6 to the recirculation loop 4, which allows injecting the powder catalyst indicated by 7 into the vessel 1 to reduce nitrous oxide (N₂O) emissions in the combustion installation.

[0015] The combustion installation in circulating fluidized bed operates preferably at a temperature ranging between 750° C. and 950° C. 

1. A procedure to reduce nitrous oxide N₂O emissions in a combustion installation which includes a vessel (1) operating as a circulating fluidized bed, wherein a catalyst pertaining to the family of zeolites is injected into the vessel (1) of the circulating fluidized bed in powder form.
 2. The procedure in accordance with claim 1 , in which the catalyst injected into the vessel represents between 0.2 and 5% of the charge of solids circulating in the vessel.
 3. The procedure in accordance with claim 1 , in which the catalyst has an average diameter ranging between 0.05 and 2 mm.
 4. The procedure in accordance with claim 1 , wherein said catalyst is injected into the vessel (1) through a recirculation loop (4) connecting the bottom of a separator (3) to the bottom of the vessel (1).
 5. An installation for the embodiment of the procedure in accordance with claim 1 , including the vessel (1) which operates as a circulating fluidized bed, a top of the vessel being connected to a particle separator (3) in which the bottom, which recovers the largest solid particles (2) present in combustion smoke, is connected to a bottom of the vessel (1) through a recirculation loop (4) equipped with a siphon, wherein a silo (5), in which a catalyst based on crushed zeolites is stored, is connected through a transport system (6) to the recirculation loop (4). 